1. Field of the Invention
This invention relates to a method for polishing and finishing an optical quality surface with a structured or three-dimensional abrasive article which does not require an external loose grain abrasive slurry or gel.
2. Discussion of the Related Art
The grinding and polishing of optical quality surfaces are important processes in producing acceptable surfaces on optical components such as lenses, prisms, mirrors, and the like. For instance, the majority of today's population requires some form of corrective eye wear, and for many, this includes corrective eyeglasses.
Every prescription eyeglass lens, including those sunglasses and safety glasses, must be individually prepared for the specific customer. While plastic lenses may be the primary material which is polished, the processes described herein are also contemplated to be applicable to other optical quality materials, such as glass. One customary method of preparing an optical lens, namely, an ophthalmic lens, for the end user consists of several steps which include grinding, fining, polishing and hard coating. Each optical lens begins as a blank lens, into which the desired curvature is imparted by the series of steps.
As indicated above, a conventional production of a smooth finished lens surface involves three basic operations. The first step is rough grinding for curve generation using a tool having a preformed, curved surface which is plated or impregnated with diamond, tungsten carbide, or other super-hard particles of the desired grit size. This tool is used to generate the desired radius or radii of curvature onto the lens form. The resultant lens surface usually is of the approximate curvature required, but is neither precise enough nor smooth enough to directly polish to the desired, final state. The next step, called "fining", in general, involves a preliminary finishing of the coarse ground surface to reduce deep scratches and provide a substantially smooth although not polished surface. The last step, called the "polishing" step, in general, involves fine grinding to remove scratches and provide a smooth, finished, or, in the case of an optical component, an optically clear surface which can receive a scratch-resistant hard coating, which is typically a curable polymer coating. This invention is concerned with the polishing step.
In somewhat greater detail, the fining step corrects the surface geometry of the lens to the exact requirement, and produces a surface texture that is smoothed sufficiently to enable the lens to be polished at a later stage from a generally milky, non-transparent state to a transparent state. Often, but not always, fining involves two separate fining steps as required in order to remove all previous scratches from the rough curve generation and produce a surface finish that is suitable for polishing with a loose abrasive slurry.
Abrasive products which are usually used for the fining operation are coated abrasives with aluminum oxide or silicon carbide abrasive particles for plastic lenses, or loose grain abrasives such alumina or silica carbide for glass lenses. The first fining step usually uses abrasive particles with a average particle size of 15 to 40 micrometers depending on the material and surface finish produced by the rough grinding step. The second fining step usually uses abrasive particles at least about 50% finer than the first, usually 4 to 12 micrometers. The time required for the two fining steps is usually from one minute to two minutes per step, depending on the starting surface finish, the abrasive particle size, and the desired resulting surface finish. The surface finish of the lens after the two fining steps typically can be anywhere from about 0.06 to 0.13 micrometer Ra, or an Rtm of from greater than 0.40 to about 0.90 micrometer.
Numerous attempts have been made to decrease the time necessary to obtain the desired surface finish and extend abrasive pad life during fining. For instance, U.S. Pat. No. 5,014,468 (Ravipati et al.) discloses a lapping film intended for ophthalmic applications comprising an interconnected pattern imparted into a surface coating of abrasive particles dispersed in a radiation cured adhesive binder.
Other prior abrasive articles for ophthalmic applications include single step fining pads, which are purportedly able to effect fining steps with a single pad in order to decrease time and cost. For instance, a one-step fining pad is disclosed in U.S. Pat. No. 4,644,703 to Kaczmarek et al. is said to provide a lens surface finish of not more than 0.25 micron AA (arithmetic average), which is understood to mean an Ra or so-called Average Roughness Height value, and not an Rtm value. Ra is the arithmetic average of the departures of the surface roughness profile from the mean line, while Rtm is defined as the mean of five individual roughness depths of five successive measuring lengths, where an individual roughness depth is the vertical distance between the highest and lowest points in the measuring length. By definition, Ra is typically measured over a significantly smaller distance than Rtm for a typical surface profile. Consistently, Kaczmarek et al. does not indicate that the lens could be successfully hard coated with a protective coating immediately after the fining operation described therein is performed.
On the other hand, U.S. Pat. No. 4,773,920 to Chasman et al. disclose a coated abrasive article useful as a lapping film where the abrasive material is formed as a dispersion of abrasive particles in a binder curable by free radical polymerization. The abrasive material is applied to a backing as a continuous coating and patterned, preferably by rotogravure roll, to provide a uniform pattern or an interconnected network of ridges with intervening grooves. The article is then cured. One use of the abrasive article exemplified in Chasman et al. is as a fining pad. A fining step, by definition, does not customarily impart the requisite final finish in the lens surface, namely, an exceedingly fine, mirror-like finish without wild or deep scratches in the surface of the lens workpiece. Therefore, further processing is typically required in this instance before the lens can receive a hard protective coating.
On the other hand, U.S. Pat. No. 4,255,164 to Butzke et al. disclose a glass fining sheet composed of a foamed liquid abrasive granule-resin coating composition. The liquid coating composition comprises a liquid curable binder, abrasive fining granules and sufficient compatible solvent to provide a coatable composition. Such a coating provides a cellular layer which releases the fining abrasive granules at a controlled rate under use conditions. Butzke et al. also describes a prior use of means to incorporate fining abrasive material into a cohesive layer so as to release abrasive material during glass grinding, but these means not having met with commercial success. Prior attempts are also mentioned to cause the binder to disintegrate, dissolve or soften to free abrasive granules, such as by adding lubricants such as stearic acid, tallow, and paraffin wax. However, these prior attempts are described as unsatisfactory as the binder material disintegrates too rapidly and problems arose with respect to unmanageable frictional heat generation.
Also, a commercially-available beaded 4 micrometer aluminum oxide abrasive article, designated 3M 356M Qwik Strip.TM. Fining Pad, manufactured by Minnesota Mining & Manufacturing, St. Paul, Minn. USA, is known for use as a fining pad which has been observed to provide a surface finish (Rtm) of about 0.44 micrometer on a polycarbonate plastic optical lens. Generally, a surface finish (Rtm) of about 0.44 micrometer is not considered a polished surface and is not a sufficient base for an acceptable hard protective coating.
After the fining step, the last step in a lens finishing procedure then becomes the polishing operation. In general, polishing conventionally has been accomplished with a loose slurry or gel containing abrasive particles which is externally introduced to the polishing site. This polishing step removes any remaining deep scratches and insures the transparent state of the lens surface, while maintaining the exact surface geometry. A typical polishing composition comprises 1 to 8 micrometers aluminum oxide particles dispersed in liquid medium such as water. The polishing time usually required with loose grain slurries is from 2 to 7 minutes per lens, again depending on the initial surface finish, the abrasive particle size, and the desired final surface finish.
There are many disadvantages associated with using an externally introduced loose grain abrasive slurry. These include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive granules and to assure a uniform concentration of abrasive granules at the grinding interface, and the need for additional equipment to prepare, handle, and also recover and recycle the abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability which requires additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the slurry eventually show undesirable wear. Further, during usage, the polishing operation is usually a very untidy process because the loose granule slurry, which is usually applied as a viscous liquid to a soft pad, splatters easily and is difficult to contain.
Understandably, attempts have been made, generally without complete success, to use coated abrasive pads to replace loose granule slurry polishing systems because of the greater ease of use of such abrasive elements. For instance, U.S. Pat. No. 4,733,502 to Braun describes a method for grinding and polishing lenses on the same machine. Fining is performed with a fining pad having abrasive fining particles fixed in a non-water soluble matrix. Then, on the same surfacing machine spindle, polishing is said to be accomplished by use of a pad having a flexible water-soluble matrix containing an abrasive polishing powder, where the polishing particles are released during polishing in the presence of a stream of water. This polishing pad is described as preferably being a pad of the type disclosed in U.S. Pat. No. 4,576,612 to Shukla et al. However, Braun does not describe the actual surface finish provided by the method in terms of Ra or Rtm, or the like, nor does Braun indicate that the polished lens could be successfully hard coated with a hard protective polymer coating even after the polishing procedure.
Further, Shukla et al. describe ophthalmic lens polishing pad where the polishing layer is produced by mixing a water soluble polyalkylene oxide/phenolic resin complex with an acrylic latex, and an alcohol slurry containing polishing particles. The polishing layer in Shukla et al. is provided as a continuous monolithic layer on a fabric substrate, or, alternatively, as a layer to completely cover or partially fill recesses in an embossed surface of a fabric substrate.
The so-called thermoplastic matrix or binder system, including the latex, as described by Shukla et al., is said to gradually dissolve during polishing to release polishing particles in a controlled manner to thus reportedly provide an acceptable glass removal rate.
As indicated before, after polishing, it is now common practice that the lens is typically coated with a hard scratch-resistant polymeric coating before the lens is put into service. The polishing step must ensure that the final finish of the lens is acceptable for such a hard coating. If any wild swirls or deep scratches are left in the lens surface before applying the hard protective coating, the coating may not be able to fill the flaws and the lens would have to be rejected. Further, even if the hard coating fills the wild swirls or deep grooves left in a lens surface, a refractive index mismatch typically occurs between the hard coating in the scratch groove and the lens to impair the optical properties of the lens.
Therefore, there is a need for a more facile durable method for polishing optical components, especially for polishing ophthalmic lenses, which obviates the need to use external abrasive slurry or gel polishing techniques.